[uml-devel] early setup

[uml-devel] early setup

[Geert Uytterhoeven]

From time to time I continue my work on gdkfb, a frame buffer device for UML
using GTK/X11...

It appears that just calling the GTK initialization routine gtk_init_check()
during kernel init causes UML to crash later, while user space is already
running. A reliable way to crash it is reading anything in /proc:

| (none):~# cat /proc/mounts
| Kernel panic: Kernel mode fault at addr 0x890856a3, ip 0x400d9304

Which is inside libgtk:

| tux$ ldd linux
|         libutil.so.1 => /lib/libutil.so.1 (0x40018000)
|         libgtk-1.2.so.0 => /usr/lib/libgtk-1.2.so.0 (0x4002a000)
|         libgdk-1.2.so.0 => /usr/lib/libgdk-1.2.so.0 (0x40148000)
|         libX11.so.6 => /usr/X11R6/lib/libX11.so.6 (0x4017c000)
|         libXi.so.6 => /usr/X11R6/lib/libXi.so.6 (0x40243000)
|         libXext.so.6 => /usr/X11R6/lib/libXext.so.6 (0x4024b000)
|         libglib-1.2.so.0 => /usr/lib/libglib-1.2.so.0 (0x40259000)
|         libgmodule-1.2.so.0 => /usr/lib/libgmodule-1.2.so.0 (0x4027a000)
|         libdl.so.2 => /lib/libdl.so.2 (0x4027d000)
|         libm.so.6 => /lib/libm.so.6 (0x40281000)
|         libc.so.6 => /lib/libc.so.6 (0x402a3000)
|         /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
| tux$

Running UML in gdb doesn't help much, UML just quits and setting a break point
doesn't work (can I single-step automatically until the PC is at 0x400d9304 so
I can get a backtrace?).

On the other hand, if I call gtk_init_check() really early (from a
__uml_setup() routine), UML works fine!

Any good reason why?  I'm using SKAS mode, since TT gave me even more headaches
(cfr. the libpthread problem from a few months ago).

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds


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Re: [uml-devel] early setup

[Jeff Dike]

On Tue, Dec 23, 2003 at 04:12:33PM +0100, Geert Uytterhoeven wrote:
> On the other hand, if I call gtk_init_check() really early (from a
> __uml_setup() routine), UML works fine!
> 
> Any good reason why?  I'm using SKAS mode, since TT gave me even more headaches

UML arranges its physical memory so that it's contiguous starting from the
beginning of the binary out to the end of its VM.  In the middle of that is the
heap.  The brk is rounded up to the nearest 4M or something.  Everything after 
that is usable kernel physical memory, everything before that is reserved and
unusable.

In order to prevent malloc from being used to allocate memory after the kernel
has taken over its memory, malloc et al are wrapped, and if the kernel is
running, the wrapper calls kmalloc instead.  So, if gtk is managing its own
heap, then it may be evading the wrapper, and causing the brk to impinge on
kernel memory.  The fact that everything works if gtk_init_check is called
early enough backs this up.  That will allocate memory and push out the brk
before UML decides where usable memory begins.

				Jeff


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